SQLite3::createAggregate()

(PHP 5 >= 5.3.0, PHP 7)

Registers a PHP function for use as an SQL aggregate function

说明

publicSQLite3::createAggregate(string $name, mixed $step_callback, mixed $final_callback[,int $argument_count= -1]): bool

Registers a PHP function or user-defined function for use as an SQL aggregate function for use within SQL statements.

参数

$name

Name of the SQL aggregate to be created or redefined.

$step_callback

Callback function called for each row of the result set. Your PHP function should accumulate the result and store it in the aggregation context.

This function need to be defined as:step(mixed $context,int $rownumber, mixed $value1[,mixed$...]): mixed$context

NULL for the first row; on subsequent rows it will have the value that was previously returned from the step function; you should use this to maintain the aggregate state.

$rownumber

The current row number.

$value1

The first argument passed to the aggregate.

...

Further arguments passed to the aggregate.

The return value of this function will be used as the$contextargument in the next call of the step or finalize functions.$final_callback

Callback function to aggregate the "stepped" data from each row. Once all the rows have been processed, this function will be called and it should then take the data from the aggregation context and return the result. This callback function should return a type understood by SQLite(i.e.scalar type).

This function need to be defined as:fini(mixed $context,int $rownumber): mixed$context

Holds the return value from the very last call to the step function.

$rownumber

Always0.

The return value of this function will be used as the return value for the aggregate.$argument_count

The number of arguments that the SQL aggregate takes. If this parameter is negative, then the SQL aggregate may take any number of arguments.

返回值

Returns TRUE upon successful creation of the aggregate,或者在失败时返回FALSE.

范例

max_length aggregation function example

以上例程会输出：

int(5)

In this example, we are creating an aggregating function that will calculate the length of the longest string in one of the columns of the table. For each row, themax_len_stepfunction is called and passed a$contextparameter. The context parameter is just like any other PHP variable and be set to hold an array or even an object value. In this example, we are simply using it to hold the maximum length we have seen so far; if the$stringhas a length longer than the current maximum, we update the context to hold this new maximum length.

After all of the rows have been processed, SQLite calls themax_len_finalizefunction to determine the aggregate result. Here, we could perform some kind of calculation based on the data found in the$context. In our simple example though, we have been calculating the result as the query progressed, so we simply need to return the context value.

Tip

It is NOT recommended for you to store a copy of the values in the context and then process them at the end, as you would cause SQLite to use a lot of memory to process the query - just think of how much memory you would need if a million rows were stored in memory, each containing a string 32 bytes in length.

Tip

You can use SQLite3::createAggregate() to override SQLite native SQL functions.

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